The present invention relates to a slit exposure type illuminating device which is mainly applicable to a color electrophotographic copier and uses a plurality of lamps such as halogen lamps.
In a prior art illuminating device for a color electrophotographic copier, a plurality of lamps are arranged in a horizontal array and are individually mounted on independent reflectors. This kind of arrangement has various drawbacks left unsolved, as follows. The lamps have to be accurately oriented in the X, Y and Z directions one by one such that the centers of their filaments are aligned with predetermined adequate positions. The lamps are usable with a predetermined kind of illuminating unit only. Since current is fed to the lamps by a bundle of wires, not only the operability is poor but also the lamps are susceptible to vibrations and, therefore, unfeasible for rapid movement. For these reasons, the prior art illuminating device is poor in productivity resulting in disproportionate cost, and the whole illuminating unit has to be replaced when any of the lamps fails because an irregular illuminance distribution cannot be compensated for by a lamp only. This is undesirable from the maintenance and economy standpoint. In addition, the device is limited in durability because its lamp harness and lamp bracket are limited in mechanical strength and apt to become loose.
Further, in a prior art color electrophotographic copier, two illuminating units of the same or different kind and each implemented with halogen lamps, fluorescent lamps or the like are individually disposed before and after a slit exposing surface. To adjust the amount of exposure for color separation, the illuminating units are controlled to turn on their lamps either independently or simultaneously or, alternatively, the voltage applied to the respective lamps is varied. A problem with the turn-on control scheme is that numerous structural elements are needed resulting in an increase in cost, poor reliability, and low illumination efficiency. On the other hand, the voltage control scheme is disadvantageous in that the adjustable range of light amount is narrow due to the variation of color temperature of light. Besides, in a color copier of the type having a plurality of slit exposing sections and using a blue, a green and a red fluorescent lamp, the illuminating device is extremely complicated in construction.
It has been customary to locally frost the bulb of a lamp with the intention of diffusing light and, thereby, eliminating ripples which would entail an irregular illuminance distribution on an original document to be copied. However, locally frosting the bulb of a lamp is time-consuming and costly.